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Why following seas create the perfect conditions for danger to ships
I’ve noticed my garden is crawling with lizards. It wasn’t something that happened overnight, but in the last few years, it feels like whenever I go outside, there’s always a little reptile scuttling out of the way to hide in the bushes. It turns out these little critters are European Wall Lizards, typically found in a much drier Mediterranean climate. Yet my city of Victoria is in a coastal rainforest climate – so what’s going on?
How they got here in the first place isn’t necessarily a mystery. There have been several documented cases of releases of these particular animals in my area over many decades. But why are they still here, and growing?
One reason is that Victoria is not necessarily so far from a Mediterranean climate. After all, it has the highest average temperature throughout the year in Canada. And though we are generally in a coastal rainforest climate, there are quite arid spots due to microclimate effects, such as rainshadow from nearby mountains. But a similar environment isn’t the only factor – food source and predation are other things that can control how successful an animal can be. But as an invasive species, they don’t have any natural predators, making it easy for them to stick around long enough to scrounge for food and multiply. In short, these lizards have the perfect conditions to thrive.
In this case, the perfect conditions line everything up for the benefit of these lizards. But things don’t always line up to create a benefit. Far from it, you can certainly get the perfect conditions for a disaster, too. And in the world of ship motion prediction, there’s no shortage of perfect conditions that can put you at serious risk. A following sea is one of those cases where you get the perfect conditions to put the vessel at serious risk. In this article, we’re going to talk about why a following sea is so dangerous to ships.
What we’re going to cover is:
- degrading steering control
- surging
- broach and capsize
First, we’re going to talk about loss of steering control.
Go with the flow and you’ll lose control
Ships controlled by steering gear, like rudders, rely on relative speed in water to work. As the flow passes over the rudder, strong lift forces develop that help to steer the ship. In fact, the faster a ship moves, the exponentially stronger these steering forces are, and the easier it is to maneuver and keep things in control. So how do ocean waves affect the rudder?
The key is in the circular flow inside ocean waves
Underneath all surface ocean waves, the flow moves in orbital motions. Generally, the flow moves in the direction of the wave at the crest, and the flow moves backward in the trough. On top of this, these circular water flows are stronger near the wave crests and taper down significantly in the water column. The effect is a general average pushing impact in the direction of the wave over time.
This means that in a following sea condition, the wave crests tend to eat away at the relative velocity developing at the steering gear. And as the relative velocity drops, the forces drop dramatically, hobbling the rudder’s ability to steer the ship. Because of the orbital motion of flow, it’s also a dynamic effect, meaning the steering force can fluctuate, too, making it a challenge to maintain consistent control. Now, the effects on the steering gear are one detail, but it’s not the only one. There can be body forces that act on the hull in a following sea that you wouldn’t see in other conditions. This brings us to the second point on surging.
Wave surge might sound fun, but for ships, it’s hazardous
In a following sea, when the wave crest velocity approaches the ship speed, the surge velocity can be amplified. If the wave is steep enough, it can also catch the vessel and cause a continuous surging speed. This forward surging speed with the wave direction means the relative speed at the rudder drops to extremely low levels, temporarily eliminating steering control forces. So the ship is at risk because of the loss of steering control – the ship can’t avoid a collision, or it might drift to a more problematic heading where ship motions are worse, putting crew and passengers at risk of injury, or equipment at risk of damage. This brings us to the third and final point on broaching and capsizing.
Loss of control can lead to a broach, and worse
Destabilizing hydrodynamic forces are always lurking around. Usually, when the ship is in calm seas and following a steady course, they are vanishingly small. So, in ideal conditions, these destabilizing forces don’t get out of control because the helm and rudder keep things working smoothly. The problem is that once you’ve lost rudder control in a following sea, you’re at risk of broaching. While at speed, a slight drift in yaw will cause these destabilizing hydrodynamic forces to rear their ugly heads and push the ship even further away. This is what causes a broach, or a sudden uncontrolled change in heading, while in waves. And depending on how severe it is, this can cause a capsize.
Ships lean over, or heel, when they turn
It’s a dynamic effect, so the amount of heel depends on the shape of the hull and height of the center of gravity, as well as vessel speed and the sharpness of the turn. The sharper and faster a broach turn is, the more danger there is to heeling over enough to capsize. Yet even if there isn’t a capsize from the broach, there will still be more hazards. The broach itself, while abrupt and jarring, doesn’t necessarily cause a capsize directly. Yet it can also leave you in an exposed condition.
The worst case scenario for motions at sea for most ships, like monohulls, is a beam sea
Beam seas are typically when many ships experience the most significant roll motions, and are when they are most likely to capsize. After a severe broach, a boat can suddenly find itself in a beam sea condition, as ocean waves that were previously following are now bearing down on it. The ship, vulnerable in beam seas, is then at risk of capsizing if the waves are big enough. Capsizing is one of the worst case scenarios for a vessels at sea, almost certainly injuring people on board, along with a substantial risk of loss of the ship. Smaller systems, such as uncrewed vehicles, may not have such severe consequences, but without a self-righting capability, they will be incapacitated without intervention.
But how likely is a broach and capsize event?
Just because a ship or vehicle is in a following sea, it doesn’t mean there’s complete certainty that such a disaster will happen. A lot of things need to line up for these very complex effects to take place – the waves need to be big enough, they have to be at a period that gets close to matching the ship speed, and the size and characteristics of the ship all factor into the odds of something like this happening. In addition, a course or speed change can avoid the problem altogether. Yet awareness of what can happen is the first step. Detecting and anticipating this may be a challenge for an uncrewed vehicle or an inexperienced crew. Yet naval architects can also design a ship to handle more severe sea states by increasing vessel power and steering capability to ensure safe operation in such conditions.
Let’s look at an example that illustrates this.
A generic 5m uncrewed vehicle was configured in ProteusDS. The small vehicle moves at a relatively slow speed of 2 kn in a following sea condition, with rudder control commanded to maintain a constant heading. In the screenshots below, the green line shows the history of the position of the vehicle through time. In a relatively mild sea state of 1m, 5s, the vehicle has little trouble maintaining course. As the sea state intensity increases significantly to 5m, 9s, you can see broaching is common, and the vehicle struggles to follow the commanded heading.
It’s summary time
We covered several facets of how following seas endanger ships, and it’s time to sum up. Following seas, where ocean waves move in the same direction as a ship, are risky for several reasons. Waves following the vessel or vehicle tend to erode and fluctuate the control forces of the steering gear, such as a rudder. This hinders control, generally speaking. Waves that are large enough and have a similar speed to the vessel put it at risk of significant surging motion, or even surfriding, causing temporary loss of steering control. If steering control is lost, destabilizing hydrodynamic effects can grow, leading to an abrupt change in heading, called a broach. In the worst case, this can put the ship or vehicle at risk of capsize because of the ensuing beam wave condition. Yet it’s possible to control these risks operationally, or even through careful design. Nevertheless, it’s a risk that can lurk around, whether you like it or not, much like a horde of European Wall Lizards hiding in the bushes around your house.
Next Step
The Generic 5m uncrewed vehicle hull was generated in a few minutes using the Orca3D Hull Assistant in Rhino. Click the image below to see how to quickly create parametric hulls for large ships or small vehicles in this video tutorial from our YouTube channel.
